Method and Apparatus for Determining Kinematic Viscosity Through the Transmission and Reception of Ultrasonic Energy

ABSTRACT

An apparatus for determining a fluid&#39;s kinematic viscosity from ultrasonic energy that has passed through the fluid of unknown viscosity along an acoustic path of known length. A computer of the apparatus determines a characteristic frequency of a received electrical signal associated with the ultrasonic energy and measures the fluid&#39;s velocity of sound. The kinematic viscosity of the fluid is determined by the computer on a continuous basis based on the characteristic frequency and the sound velocity. A method for determining a fluid&#39;s kinematic viscosity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 14/171,394filed Feb. 3, 2014, now U.S. Pat. No. 9,448,150, incorporated byreference herein.

FIELD OF THE INVENTION

The present invention is related to determining the kinematic viscosityof an unknown fluid using ultrasonic energy traveling along a known pathlength. (As used herein, references to the “present invention” or“invention” relate to exemplary embodiments and not necessarily to everyembodiment encompassed by the appended claims.) More specifically, thepresent invention is related to determining the kinematic viscosity ofan unknown fluid using ultrasonic energy traveling along a known pathlength, where from the ultrasonic energy, an associated characteristicfrequency and sound velocity of the fluid is derived to determine thekinematic viscosity.

BACKGROUND OF THE INVENTION

This section is intended to introduce the reader to various aspects ofthe art that may be related to various aspects of the present invention.The following discussion is intended to provide information tofacilitate a better understanding of the present invention. Accordingly,it should be understood that statements in the following discussion areto be read in this light, and not as admissions of prior art.

In petroleum and other pipelines, there exists a need to measure certainproperties of the fluid, which generally is flowing. In particular, ameasurement of viscosity (either absolute or kinemustmatic) is neededto: (a) differentiate fluids, (b) detect the interface between twodifferent fluids, (c) characterize pressure gradients in a pipeline forpurposes of leak detection and locations, (d) determine when a change orinterface between fluids occur, and (e) determine the required amount ofdilution agent to meet the maximum viscosity limit set by the pumpingpower and pressure rating of the pipeline.

Currently available means for these measurements are complex, expensiveand sometimes unreliable. For example, viscous forces are sometimesmeasured by vibratory systems. For these means and most others, a bypassline is necessary to direct a fraction of the flowing fluid to the meansof measurement. The bypass can become obstructed with waxes or otherelements carried by the flowing fluid. In addition, the moving parts ofsuch measurement means can create maintenance and calibration problems.Many pipeline operators take grab samples of the flowing fluid todetermine density and viscosity, because the accuracy and reliability ofthe on-line means do not meet their requirements. The expense of thesampling procedure is obvious. In addition, the procedure deprives thepipeline operator of the ability to monitor and control the propertiescontinuously and in real time.

BRIEF SUMMARY OF THE INVENTION

The present invention uses the transmission and reception of acousticsignals that transit through an unknown fluid. The viscosity is computedwith signal analysis in the frequency domain. The center frequency ofthe received signal is determined to be a function of the viscosity ofthe fluid medium and this frequency is compared with the received signalfrequency measured in the laboratory for a product of a known viscosity.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

In the accompanying drawings, the preferred embodiment of the inventionand preferred methods of practicing the invention are illustrated inwhich:

FIG. 1 is a graph showing attenuation due to viscous losses.

FIG. 2 shows a transfer function of a transducer waveform.

FIG. 3 shows viscous attenuation and unfiltered waveform together.

FIG. 4 shows net waveform bandwidth after different amounts of viscousattenuation.

FIG. 5 is a graph of the net waveform's center frequency calculated fromthe median frequency for each of the responses of FIG. 4.

FIG. 6 is a graph of log (viscosity x path length) USFP.

FIG. 7 is a block diagram of the apparatus of the present invention.

FIG. 8 is a flow chart of the calculation of Reynolds Number and MeterFactor Correction for a chordal ultrasonic meter.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like reference numerals refer tosimilar or identical parts throughout the several views, and morespecifically to FIG. 7 thereof, there is shown an apparatus 100 fordetermining a fluid's kinematic viscosity. The apparatus 100 comprises atransmitter 1 and transmitting transducer 2 capable of converting aninput pulse from the transmitter 1 into ultrasonic energy. The apparatus100 comprises a receiving transducer 4 and electronic amplifier 5 whichconvert some of the ultrasonic energy that has passed through the fluidof unknown viscosity into an electrical signal. The ultrasonic energy iscreated by the transducer follows an acoustic path 3 which is made topass through the fluid of unknown viscosity along a path 3 of knownlength. The apparatus 100 comprises a computer 20 which determines acharacteristic frequency of the received electronic electrical signaland measures the fluid's velocity of sound. The computer 20 determineskinematic viscosity of the fluid on a continuous basis based on thecharacteristic frequency and the sound velocity, where the kinematicviscosity is formed from the fluid's bulk viscosity and shear viscosity.

The bulk viscosity has a fixed relationship with the shear viscosity.The computer 20 may combine the characteristic frequency and soundvelocity with similar data measured for a fluid of different viscosityand sound velocity to form a frequency parameter. The data may have beenmeasured as part of a calibration process of the apparatus 100.

The computer 20 may determine a logarithm to an arbitrary base of aviscosity-path 3 length product for the fluid of unknown viscosity fromthe frequency parameter and from the data taken during calibration ofthe apparatus 100. The frequency parameter and the data may be used bythe computer 20 to establish a slope and offset of the logarithm to thearbitrary base of a viscosity-path 3 length product versus frequencyparameter relationship. The computer 20 may subtract a logarithm of thepath 3 length from a logarithm of the viscosity path 3 length product toyield a result, and the computer 20 takes an antilog of the result,thereby determining the kinetic viscosity of the fluid. The computer 20may include a received signal digitizer 6, signal detection and timemeasurement logic 7, sound velocity C and received frequency calculationmodule 8, frequency parameter (FP) calculation module 9, log (VL)calculation module 10, and viscosity calculation module 11.

The present invention pertains to a method for determining a fluid'skinematic viscosity. The method comprises the steps of triggering atransmitter 1 which generates an electrical transmit pulse. There is thestep of commencing counting of timing pulses from a digital clock 30simultaneous with the transmit pulse transmission. There is the step ofapplying the transmit signal via electrical cables to an ultrasonictransmitting transducer 2. There is the step of causing the transmittingtransducer 2 with the transmit pulse to produce an ultrasonic pressurepulse having a limited number of sinusoidal cycles. The ultrasonictransmitting transducer 2 is mounted such that ultrasonic energy itproduces travels through the fluid whose viscosity is to be measuredalong a path 3 of known length. There is the step of transferring at theend of the path 3 some of this energy to a receiving transducer 4. Thereis the step of converting with the receiving transducer 4 some of theultrasonic energy into an electrical receive pulse. There is the step ofamplifying by a receiver 5 the receive pulse whose frequency response issuch that the energy content of all frequencies that are present in thereceive pulse is preserved. There is the step of digitizing in adigitizer 6 the receive pulse. There is the step of reconstructing thereceive pulse in digital format 7. There is the step of detecting a timeof the receive pulse's arrival by signal detection logic. There is thestep of measuring with a computer 20 transit time through the viscousfluid of the pressure pulse and transit time through non fluid media ofthe energy transmission path 3, including electrical delay of the cablebetween the transmitter 1 and the transmitting transducer 2, mechanicaldelay associated with an interface between the transmitting transducer 2and the fluid, mechanical and cable delays associated with the receivingtransducer 4 and electronic delays of the receiver 5. There is the stepof calculating the fluid's sound velocity as a quotient of the path 3length L through which the ultrasonic energy has traveled and thetransit time of the ultrasonic energy in the fluid. There is the step ofcalculating a frequency parameter (FP) from the sound velocity andfrequency measurements of the received ultrasonic energy pulse. There isthe step of determining a Logarithm of a viscosity path 3 length productLog (vL) with the computer 20 from the Frequency Parameter with a loglinear characteristic. There is the step of calculating with thecomputer 20 a kinematic viscosity v by subtracting the logarithm of thepath 3 length from the Log (vL) to obtain a reminder, then taking anantilog of the reminder.

In the operation of the invention, kinematic viscosity of a flowing orstationary fluid is measured, a property not readily and reliablymeasured on a continuous basis.

Examples of the use of such a measurement may be for any or all of thefollowing purposes:

-   -   Linearization of flow meters    -   Flowmeters such as turbine meters, helical turbine meters and        ultrasonic meters may have Reynolds number dependent calibration        curves. Knowledge of fluid viscosity allows the linearization of        such meters, effectively widening their range and improving        their accuracy.    -   Detection of feedstock properties for purposes of blending    -   In oil processing and other industries, real time control of        additive injection to achieve a desired viscosity requires        knowledge of the viscosity of the incoming feedstock as well as        knowledge of the viscosity of the blend after injection of the        additive.    -   Detection of viscosity of products entering a pipeline to        establish pumping power surcharges    -   The frictional losses in a pipeline are a direct function of the        product viscosity. Pumping power is a major pipeline operating        expense (the pumping power for a 30 inch diameter 1500 mile        pipeline can approach 1100 MWe) and a batch surcharge based on        product viscosity would be an equitable means for allocating        energy costs.    -   Measurement of product viscosity for purposes of pipeline leak        detection modeling    -   Calculation of compressibility effects and determination of leak        location require an accurate characterization of the pressure        drop-flow characteristics of the pipeline and its products. The        viscosity is a key variable in this characterization.

Unique requirements and properties of the present invention may include:

-   -   Excitation with a pulse signal to provide ultrasonic energy.        Ultrasonic energy has a broad and defined frequency spectrum.    -   A receiver 5 having a broad and defined pass band.    -   A detection of received ultrasonic signals and measurement of        center frequencies, for purposes of determining the        characteristic frequency of the received signal. This        measurement of frequency could be done either in the frequency        domain, e.g., Fourier transform or in the time domain, e.g.,        signal period.    -   Measurement of the sound velocity of the medium having unknown        viscosity.    -   Calibration by measuring the received frequencies in at least        two different media of known or independently measured        viscosities, one medium typically having low viscosity and the        other having high viscosity.    -   Measurement of the sound velocities of the media used for        calibration.

The principle of the present invention uses the transmission andreception of acoustic signals that transit through an unknown fluid. Theviscosity is computed by signal analysis in the frequency domain. Thecenter frequency of the received signal is determined to be a functionof the viscosity of the fluid medium and this frequency may be comparedwith the received signal frequency measured in the laboratory for aproduct of a known viscosity.

The physical principles of the system will be described first. Thesystem consists of:

-   -   (1) A transmitter 1 which supplies excitation to a transmitting        transducer 2,    -   (2) A transmitting transducer 2    -   (3) A fluid medium (When calibrating the device, two reference        fluids are used, one typically has low kinematic viscosity (˜1        cSt) and the other has a higher kinematic viscosity (2.5 to over        3000 cSt).    -   (4) A receiving transducer 4    -   (5) A receiver which amplifies and digitizes the received        ultrasound. The receiver 5 may be equipped with (a) a low        frequency stop filter, to eliminate noise at frequencies below        the frequency range of interest for the measurement and (b) a        high frequency stop filter.    -   (6) A computer 20

Sound Attenuation in a Viscous Medium

The intensity of an acoustic beam is attenuated in a viscous mediumaccording to the following (Kinsler and Frey, “Fundamentals ofAcoustics”, Chapter 9, incorporated by reference herein):

Av=exp(−2αX)   Equation (1)

Where: Av=Viscous attenuation factor

-   -   α=Attenuation coefficient    -   X=Acoustic path length in the medium

The number 2 appears in the expression because it describes the loss ofacoustic energy (as opposed to pressure).

From the analysis of the cited reference, the following expression forthe attenuation coefficient, a can be derived:

α=2/3ω²v/C³   Equation (2)

Where: ω=Angular frequency of the ultrasound

-   -   ω=2πf, where f is the ultrasound frequency    -   v=Kinematic viscosity    -   C=Sound velocity in the medium.

According to the cited reference, the kinematic viscosity of Equation(2) attenuates the ultrasound because of the combined effects of:

-   -   (a) The shear viscosity. At the pipe wall the axial velocity of        the flowing fluid is diminished to zero from a representative        free stream velocity by the friction between adjacent layers of        the flowing fluid traveling at different axial velocities. The        shear viscosity characterizes the frictional losses of flowing        fluid in smooth pipe and determines the shape of the velocity        profile, particularly near the pipe wall.    -   (b) The bulk kinematic viscosity. The previously cited reference        also describes a second component of viscosity, which engenders        losses in ultrasound transmitted through a viscous medium. This        component has to do with the ultrasonic compression and        expansion of the fluid at the molecular level.

The technique described herein finds the total kinematic viscosity—thecombination of shear and bulk effects. As long as the bulk viscositybears a fixed relationship to the shear viscosity (as it does in manyfamilies of fluids), the viscosity determined by the technique will be avalid index for the shear viscosity.

Rewriting Equation (1) below and substituting Equation (2) into it, theattenuation term becomes a function of frequency (where: ω=2πf):

$\begin{matrix}{{{Av}(\omega)} = {\exp\left( {- \frac{4\omega^{2}{vX}}{3C^{3}}} \right)}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

Examples of the attenuation versus frequency expected due to viscouslosses is shown for a 12 inch path length in FIG. 1.

Excitation/Transmission

The excitation of the transmitting transducer 2 is provided by animpulse signal with broad spectrum bandwidth. The use of an impulse isnot a requirement, in fact, using a rectangular pulse is typical, butfor the sake of simplifying the math here, an impulse is used (e.g.,frequency response is 1 for all frequencies).

Transmitting and Receiving Transducers

The transmitting and receiving transducers 2, 4 are nominally identicaland therefore reciprocal. The transfer function of the transducer properis characterized as the current response to a voltage impulse input toan R, L, C circuit where the R describes the energy transduced andtransmitted to the adjacent medium, L, the inertial mass of thetransducer material and C its stiffness; see Kinsler and Frey,previously cited. The use of reciprocal transducers is not a requirementbut is shown to simplify the analysis.

This simplified model leads to the transfer function of a classicunderdamped second order system, expressed as a function of the angularfrequency ω normalized to the transducer resonant frequency, ω₀

$\begin{matrix}{{G({jw})} = \frac{j\; 2{\zeta\omega\omega}_{0}}{{- \omega^{2}} + \omega_{0}^{2} + {j\; 2{\zeta\omega\omega}_{0}}}} & \left. 4 \right)\end{matrix}$

Here G(jw)=transducer's transfer function

-   -   ω₀=natural frequency of the transducer    -   ζ=damping coefficient for the transducers    -   j=square root of −1

The transfer function for the combination of the transmitting transducer2 and the receiving transducer 4 is the square of the transfer functionof equation 4:

$\begin{matrix}{{G({jw})}^{2} = \frac{- \left( {2{\zeta\omega\omega}_{0}} \right)^{2}}{\left( {{- \omega^{2}} + \omega_{0}^{2}} \right)^{2} - \left( {2{\zeta\omega\omega}_{0}} \right)^{2}}} & \left. 5 \right)\end{matrix}$

An example of such a transfer function is shown in FIG. 1. For thisfigure, the damping coefficient was taken as 0.5 and the centerfrequency is taken as 1 MHz.

The Receiver Input Circuit

The shape of the received signal is also affected by a seriescapacitance—resistance circuit at the receiver 4 input, which couplesalternating electrical current from the receiving transducer 4 to thefirst stage of the receiver 4. For simplification, the receiver 4 isalso assumed to have a flat passband (e.g., frequency response is 1 forall frequencies).

Analytical Results—Frequency Shift as Function of Viscosity

The transfer function of the transmitter-transducer-attenuatingfluid-transducer-receiver input circuit is the product of the transferfunctions of the individual elements:

$\begin{matrix}{{H(w)} = {\frac{- \left( {2{\zeta\omega\omega}_{0}} \right)^{2}}{\left( {{- \omega^{2}} + \omega_{0}^{2}} \right)^{2} - \left( {2{\zeta\omega\omega}_{0}} \right)^{2}}{{Av}(\omega)}}} & \left. 6 \right)\end{matrix}$

FIG. 3 shows the calculated transfer function of the acoustic waveformwithout viscous attenuation and the attenuation produced by a range ofviscosity-path length conditions (The viscosity path length product isused because the attenuation of the energy is a function of the productof the attenuation factor (which in turn contains sound velocity,frequency and viscosity) and the path length. For the graph, the pathlength is 12 inches and the sound velocity is 50,000 in/s.)

The net waveform response including attenuation is the product of thewaveform's transfer function without viscous attenuation and thetransfer function characterizing the viscous attenuation for thespecific conditions under investigation. FIG. 4 provides examples of thenet bandwidth.

The net waveform's center frequency has been calculated from the medianfrequency for each of the responses of FIG. 4 is graphed in FIG. 5.Another method would be to take the inverse transforms of the transferfunctions of FIG. 4. For the cases where viscous attenuation issignificant, the procedure is reasonably accurate, since the compositetransfer functions are approximately symmetrical about the peak.

Experimental Results

Data for both low and high viscosity oils have been analyzed to confirmthat the means disclosed herein can accommodate a wide range ofviscosities.

Data were drawn from two sources:

(a) Data set 1: Measurements in the calibration lab of frequency shiftson an 8 inch ultrasonic meter having 1.6 MHz transducers, a 20 inchultrasonic meter having 1.0 MHz transducers, and a 24 inch ultrasonicmeter having 1.0MHz transducers. 16 separate measurements of frequencyshift were made for viscosities ranging from 1 cSt (water) to about 86cSt.

(b) Data Set 2: Measurements made in a special test rig which allowedthe distance between transmitting and receiving transducers 2, 4 to bevaried. The rig was filled with liquid water (for reference) and withoils having viscosities of 300, 1120 and 4000 cS. Distances were variedfrom 4 inches to 45 inches except for the heaviest oils, where a weakreceived signal limited the maximum distance to 25 inches.

The technique described here defines a Frequency Parameter FP that isbased on the analysis of the previous section and is closely correlatedwith the fluid viscosity. The form and preliminary values of theconstants of the correlation between the frequency parameter and theviscosity are given herein. The values of the constants of thecorrelation can be enhanced during the factory acceptance tests andcalibrations.

To enhance the accuracy of the viscosity determination, the FrequencyParameter—the independent variable of that determination—shouldincorporate those variables that an ultrasonic flowmeter can measure inreal time in the field and that can affect viscous attenuation, as wellas those variables that can be measured in factory acceptance tests(e.g. received signal frequency without significant viscousattenuation).

Equation (6) determines the frequency change brought about by theattenuation of transmitted ultrasound in a viscous medium, as againstthe attenuation in a medium of low viscosity. The equations lead to thefollowing definition for the frequency parameter FP:

FP=(1/v ₀) (C ₀ /C)³(f/f ₀)²   Equation (4)

The zero subscripted variables in the Frequency Parameter are thosemeasured with water (reference fluid); the variables without subscriptsare measured in real time. If the viscosity is measured as an adjunct ofa flow measurement these variables are measured concurrent with thevolumetric flow measurement of the medium having unknown viscosity.

The Frequency Parameter has been calculated for the data sets referencedabove, a total of 41 data points. The logarithms, to the base 10, of theviscosity—path length products (Again, the frequency parameter iscorrelated with the product because the attenuation of the energy is afunction of the product of the attenuation factor (which in turncontains sound velocity, frequency and viscosity) and the path length)measured for those data sets are plotted against the FrequencyParameters for the data in FIG. 6. A logarithm is selected for theordinate because of the form of Equation (1), which shows the viscousattenuation to be an exponential function of the viscosity-path lengthproduct. The fit of these data can be used to determine an unknownproduct viscosity. The Frequency Parameter, whose inputs are measuredconcurrently with the measurement of the flow of a fluid having anunknown viscosity and, for the 0 subscripted variables, in FAT tests,would be entered into the data fit algorithm to determine the log to thebase 10 of the viscosity path length product. The base of the logarithms(10) would be raised to the numerical value of the log (i.e. the power)found using the fit, and that result divided by the (known) path lengthto determine the viscosity.

From the data of FIG. 6 the following conclusions are drawn:

1. The linear fit of the two variables shown in the figure correlatesthe data well. The extent of the fit is excellent—kinematic viscositiesrange from 2.5 to 4000 cSt, and comprise 6 different oils.

2. The fit is expressed as follows:

Log₁₀(νL _(path))=m×(FP)+b.

Where m=−2.576

-   -   b=5.0756

3. One standard deviation of the data about the linear logarithm fit ofFIG. 6 is about ±0.18 (units are Log to the base 10 of theviscosity-path length product).

4. The departures of the individual data sets from the fit are tabulatedbelow:

Average deviation Nominal viscosity Number of data from fit of all dataof test oil cS points points, Log (ν L) 2.5 2 +0.05 12 8 +0.001 80 6+0.009 300 9 −0.095 1120 9 −0.096 4000 7 +0.22

The large deviation of the 4000 cS data set may be due to the weaknessof the received signal with this oil. The weak signal, in combinationwith coherent noise, will produce distortion, which can introduce biasesin the measurement of the received signal frequency and therefore in theFrequency Parameter. If the 4000 cS data are excluded the standarddeviation of the fit is reduced to ±0.15 (log to the base 10 units).

Description of the Apparatus

FIG. 7 shows an example of the apparatus 100 for carrying out theviscosity measurement as described herein. The figure shows the mostrudimentary embodiment; more sophisticated embodiments will also bedescribed.

The measurement is initiated by triggering a transmitter 1) whichgenerates an electrical pulse. Simultaneous with the transmission of thepulse, counting of timing pulses from a digital clock 30 will commence.The timing pulse count will stop at later times in the process as willbe described below.

The transmitter pulse will be connected via electrical cables to anultrasonic transducer 2. Electrical connections are shown as solid linesin FIG. 7. Dashed lines indicate mechanical connections. Hatched linesindicate related digital computations. The application of the pulse tothe transmitting transducer 2 causes that device to produce anultrasonic pressure pulse having a limited number of sinusoidal cycles(3 to 5). The period of the sinusoid is set by the interaction of thebroad spectrum of the pulse and the “natural period” of the transmittingtransducer 2. The natural period of the transmitting transducer 2 is setby its dimensions and the speed of sound in the transducer material.

The ultrasonic transmitting transducer 2 is mounted such that theultrasonic energy it produces is transferred to the fluid whoseviscosity is to be measured. The ultrasonic energy travels through thefluid along a path 3 of known length, L.

At the end of the path 3, some of this energy is transferred to areceiving transducer 4, having characteristics similar to thetransmitting transducer 2. The receiving transducer 4 converts some ofthe ultrasonic energy into an electrical pulse.

The path 3 through the viscous fluid and each interface in the acousticpath 3 diminishes the electrical energy of the received ultrasonicpulse. Accordingly the received signal is amplified by a receiver 5,whose frequency response is such that the energy content of allfrequencies that are present in the received signal is preserved.Because of this characteristic the receiver 5 is said to be “broadband”.

The received signal is digitized in a digitizer 6, an analog to digitalconverter employing a sample frequency higher, by order of magnitude ormore, than the natural frequency of the transducers.

The signal is then reconstructed in digital format by a computer 20 andthe time of its arrival is detected by signal detection logic 7 of thecomputer 20. Details of the detection logic itself are not shown and arenot unique to this invention and are well known. In summary, the 1stlarge half cycle of the received signal is detected when a signal ofmagnitude greater than an enabling threshold is received. The time atwhich the next zero crossing of the received signal occurs, t₁ ismeasured using the count of timing pulses that was initiated when thetransmitter 1 applied the pulse to the transmitting transducer 2. Alsomeasured is t₂, the time of the zero crossing following t₁. The t₂measurement also uses the count of timing pulses occurring betweentransmission and this zero crossing.

The times measured include not only the transit time through the viscousfluid but also the transit times through the non-fluid media of theenergy transmission path 3, such as the electrical delay of the cablebetween the transmitter 1 and the transmitting transducer 2, themechanical delay associated with the interface between the transmittingtransducer 2 and the viscous fluid, the mechanical and cable delaysassociated with the receiving transducer 4 and the electronic delays ofthe receiver 5. These last include the delay between the leading edge ofthe received signal and the zero crossing at which t₁ is measured. Thedelays in the non-fluid media should be accounted in the signalprocessing described below. The delays can be calculated or measured aspart of the calibration process by means that are not unique to thisinvention.

The digital signal processing 8 calculates the sound velocity of thefluid having unknown viscosity as the quotient of the path 3 length Lthrough which the ultrasound has traveled and the transit time of theultrasound in the fluid. The transit time of the ultrasound in the fluidis given by the difference between the measured time t₁ and sum of thedelays in non-fluid media, τ.

The signal processing 8 also calculates the dominant frequency of thereceived signal as the reciprocal of period of that signal. The periodof the received signal is taken as twice the difference between the timeof the zero crossing of the half cycle following signal detection, t₂and the time of the zero crossing of the trailing edge of the detectionhalf cycle t₁. There is no need to subtract non fluid delays from thetime measurements used to determine period because the same delays arepresent in both measurements.

In many applications the fluid for which the viscosity to be measuredwill be flowing. As a consequence some component of fluid velocity willproject onto the acoustic path 3. In such cases counterpropagation ofacoustic pulses through the fluid will be employed. Counterpropagationemploys a multiplexer to initiate acoustic pulse propagation first inone direction (such as that shown in FIG. 7) then in the other, usingwhat was previously the receiving transducer 4 as the transmittingtransducer 2 and processing the signal received on what was previouslythe transmitting transducer 2. Counterpropagation makes use of thereciprocal nature of the acoustic path 3 and the similar characteristicsof the transmitting and receiving transducers 2 and 4. It is oftenemployed in transit time ultrasonic flowmeters, because the differencein transit times in the upstream and downstream directions along anacoustic path 3 provides a measure for the fluid velocity projected ontothat path 3. In the case of this invention, using the average of transittimes t_(1up) and t_(1down) of counterpropagated pulses eliminates anyerror in sound velocity due to the presence of a non-zero fluidvelocity.

The results of the sound velocity and received frequency measurementsare used to calculate a frequency parameter FP by the FP calculationmodule 9. For this computation, many samples of the measured soundvelocity and received signal frequency may be employed to reduce errorsdue to the width of the clock 30 pulses, turbulence and randomelectrical noise that may be present in a single sample. Baseline datataken during acceptance testing and calibration of the apparatus 100(which is also considered an ultrasonic flow meter) are also employed inthe calculation of the frequency parameter. These data include aviscosity v₀ for a low viscosity fluid for which the frequency f₀ andsound velocity C₀ are also measured.

The Frequency Parameter used with a log linear characteristic todetermine the Logarithm of the viscosity path length product Log (vL) bythe log (VL) calculation module 10. The constants of the Log linearcharacteristic—the slope m and the offset b—are established during theprocess of calibrating the system in the laboratory, by measuring thehalf period of the received signal and the sound velocity for fluids ofknown viscosity.

The viscosity v is then calculated by subtracting the logarithm of thepath length from the Log (vL), then taking the antilog by the viscositycalculation module 11.

FIG. 8 is a flow chart of the implementation of the technique disclosedherein as applied to the determination of Reynolds Number.

Although the invention has been described in detail in the foregoingembodiments for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be described by thefollowing claims.

1. An apparatus for determining a fluid's kinematic viscositycomprising: a transmitter and a transmitting transducer capable ofconverting a single pulse from the transmitter into ultrasonic energy; areceiving transducer which converts some of the ultrasonic energy thathas passed through a fluid of unknown viscosity into an electricalsignal, the ultrasonic energy created by the transmitting transducerfollows an acoustic path which is made to pass through the fluid ofunknown viscosity along a path of known length; and a computer whichdetermines a characteristic frequency of a received electrical signaland which determines kinematic viscosity of the fluid based on thecharacteristic frequency, where the kinematic viscosity is formed from afluid's bulk viscosity and shear viscosity, wherein the fluid's bulkviscosity has a fixed relationship with the fluid's shear viscosity. 2.The apparatus of claim 1 wherein the fluid is flowing.
 3. A method fordetermining a fluid's kinematic viscosity comprising the steps of:triggering a transmitter which generates an electrical transmit pulse;commencing counting of timing pulses from a digital clock simultaneouswith a transmit pulse transmission; applying the transmit pulse viaelectrical cables to an ultrasonic transmitting transducer; causing thetransmitting transducer with the transmit pulse to produce an ultrasonicpressure pulse, the ultrasonic transmitting transducer is mounted suchthat ultrasonic energy the transmitting transducer produces travelsthrough a fluid whose viscosity is to be measured along a path of knownlength; transferring at an end of the path some of this energy to areceiving transducer; converting with the receiving transducer some ofthe ultrasonic energy into an electrical receive pulse; digitizing in adigitizer the receive pulse, an analog to digital converter employing asample frequency higher, by order of magnitude or more, than the naturalfrequency of the transmitting transducer and the receiving transducer;reconstructing the receive pulse in digital format; detecting a time ofthe receive pulse's arrival by signal detection logic; measuring with acomputer a transit time through the viscous fluid of a pressure pulseand a transit time through non fluid media of an energy transmissionpath, including electrical delay of the cable between the transmitterand the transmitting transducer, mechanical delay associated with aninterface between the transmitting transducer and the fluid, mechanicaland cable delays associated with the receiving transducer and electronicdelays of the receiver; calculating a frequency parameter FP from thesound velocity and frequency measurements of a received ultrasonicenergy pulse; and calculating with the computer a kinematic viscosity vas a function of the frequency parameter wherein the fluid's bulkviscosity has a fixed relationship with the fluid's shear viscosity. 4.The method of claim 3 wherein the fluid is flowing.
 5. An apparatus fordetermining a fluid's kinematic viscosity comprising: a transmitter anda transmitting transducer capable of converting a single pulse from thetransmitter into ultrasonic energy; a receiving transducer whichconverts some of the ultrasonic energy that has passed through a fluidof unknown viscosity into an electrical signal, the ultrasonic energycreated by the transmitting transducer follows an acoustic path which ismade to pass through the fluid of unknown viscosity along a path ofknown length; a computer which determines kinematic viscosity of thefluid by signal analysis in a frequency domain where a center frequencyof a received electrical signal is a function of the viscosity of thefluid where the kinematic viscosity is formed from a fluid's bulkviscosity and shear viscosity, wherein the fluid's bulk viscosity has afixed relationship with the fluid's shear viscosity.